Lithography process on high topology features

ABSTRACT

A method includes forming a first photo resist layer over a base structure and a target feature over the base structure, performing an un-patterned exposure on the first photo resist layer, and developing the first photo resist layer. After the step of developing, a corner portion of the first photo resist layer remains at a corner between a top surface of the base structure and an edge of the target feature. A second photo resist layer is formed over the target feature, the base structure, and the corner portion of the first photo resist layer. The second photo resist layer is exposed using a patterned lithography mask. The second photo resist layer is patterned to form a patterned photo resist.

This application claims the benefit of provisionally filed U.S. PatentApplication No. 61/778,261, filed Mar. 12, 2013, and entitled“Lithography Process on High Topology Features,” which application ishereby incorporated herein by reference.

BACKGROUND

In the formation of integrated circuits, the components of theintegrated circuit devices need to be patterned to form desirableshapes. A typical patterning process includes a lithography process,which includes coating a photo resist over a target layer that is to bepatterned, exposing the photo resist using a lithography mask,developing the photo resist, and using the developed photo resist toetch the target layer. As a result, the layout of the developed photoresist is transferred to the underlying layer. The photo resist is thenremoved.

There are some integrated circuits whose formation involves hightopology features. In the high topology features, the height differencebetween a high feature and a neighboring low feature may be as high asabout 100 nm to about 300 nm, or even higher. For example, FinField-Effect Transistors (FinFETs) typically have fins much higher thanthe top surface of surrounding Shallow Trench Isolation (STI) regions.In addition, gate dielectrics, polysilicon gates, and hard masks may bestacked over the fins. Therefore, the heights of these layers arefurther added to the heights of the fins. This results in a severetopography problem that incurs difficulty in the respective lithographyprocess.

When lithography processes are performed on the wafers having a hightopology, the photo resist and the Bottom Anti-Reflective Coating (BARC)need to be formed. The photo resist and the BARC suffer from thicknessnon-uniformity. For example, the BARC may be accumulated at the cornerformed between an edge of a tall feature and a top surface of a lowfeature. On the other hand, the portion of the BARC directly over thetall feature may have a very small thickness. As a result, the BARCthickness is not uniform throughout a respective wafer.

To reduce the thickness uniformity, various approaches were taken. Forexample, a negative tone photo resist may be used to fill the gaps nextto tall features. A positive tone photo resist is then used to definethe desirable patterns. This process, however, incurs increased processcost since an additional negative tone photo resist needs to be formed,and an additional lithography process and lithography mask are needed topattern the negative tone photo resist. In addition, in these processes,damages may occur to the portions of the tall features that are notsupposed to be etched, while residues of the low features may beundesirable left after the patterning.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantagesthereof, reference is now made to the following descriptions taken inconjunction with the accompanying drawings, in which:

FIGS. 1 through 8 are cross-sectional views of intermediate stages of alithography process in accordance with some exemplary embodiments; and

FIG. 9 illustrates a cross-sectional view of a wafer including aplurality of features having different heights in accordance with somealternative exemplary embodiments, wherein the formation of aself-aligned photo resist results in the smoothing of the top surface ofthe respective wafer.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments of the disclosure are discussedin detail below. It should be appreciated, however, that the embodimentsprovide many applicable concepts that can be embodied in a wide varietyof specific contexts. The specific embodiments discussed areillustrative, and do not limit the scope of the disclosure.

A patterning process incorporating a lithography process is provided inaccordance with various exemplary embodiments. The intermediate stagesof performing the patterning process are illustrated. The variations ofthe embodiments are discussed. Throughout the various views andillustrative embodiments, like reference numbers are used to designatelike elements.

Referring to FIG. 1, an initial structure of wafer 100 is formed. Theinitial structure includes base structure 20, and target feature 22 overbase structure 20. In accordance with some embodiments, base structure20 includes the features selective from, and not limited to, asemiconductor substrate, a dielectric substrate, dielectric layers,metal features (not shown), or the like. Target feature 22 is thefeature that is to be patterned (etched) in subsequent lithographyprocesses. Target feature 22 may be any of a semiconductor feature, adielectric feature, a metallic feature, and the like. In some exemplaryembodiments, Target feature 22 comprises polysilicon, and may be used toform polysilicon gates for transistors such as Fin Field-EffectTransistors (FinFETs).

Hard mask 24 may be formed over target feature 22 in some embodiments tohelp pattern target feature 22. In some embodiments, hard mask 24comprises silicon nitride, titanium nitride, tantalum nitride, or thelike. In alternative embodiments, there is no hard mask formed overtarget feature 22. The combined height H1 of features 24 and 22 may begreater than about 100 nm, and may also be between about 100 nm andabout 300 nm. It is appreciated, however, that the values recitedthroughout the description are merely examples, and may be changed todifferent values.

Photo resist 26 is applied over hard mask 24 and base structure 20. Thethickness T1 of photo resist 26 is related to height H1, and the greaterheight H1 is, the greater thickness T1 is used. In some exemplaryembodiments, thickness T1 is greater than about 50 percent, or greaterthan about 75 percent, height H1. Photo resist 26 may be applied, forexample, by spin coating. In some embodiments, photo resist 26 is apositive tone photo resist. The positive tone photo resist 26 comprisesa polymer, and is free from, or substantially free from, anycross-linking agent therein. When exposed to light, the exposed portionsof positive tone photo resist 26 are decomposed, and may be removed in asubsequent developing step. The unexposed portions of positive tonephoto resist 26 are not decomposed, and will not be removed by thedevelopment step.

In alternative embodiments, photo resist 26 is a negative tone photoresist. The negative tone photo resist 26 also comprises a polymer, andcomprises a cross-linking agent. In some exemplary embodiments, thecross-linking agent that may be comprised in negative tone photo resist26 is selected from the group consisting of amino compounds such asmelamine resins, urea resins, guanamine resins, glycoluril-formaldehyderesins, succinamide-formaldehyde resins, ethylene urea-formaldehyderesins, and combinations thereof. When exposed to light, the exposedportions of the polymer chain in negative tone photo resist 26cross-link with each other, and will not be removed in a subsequentdeveloping step. The unexposed portions of the negative tone photoresist do not cross-link, and hence are removed by the development step.

Referring to FIG. 2, an un-patterned exposure is performed on photoresist 26, wherein photo resist 26 is exposed to light 30 uniformly.Throughout the description, the term “un-patterned exposure” refers tothe exposure of a photo resist without using patterned lithography maskthat includes opaque patterns for blocking light and transparentportions for allowing light to pass through. In some embodiments, nomask is placed over photo resist 26 when the un-patterned exposure isperformed. In alternative embodiments, mask 28 may be placed over photoresist 26 and in the light path of light 30. Mask 28, however, is notpatterned, which means that the light intensity of light 30 is reduceduniformly (but not fully blocked) by mask 28.

Referring to FIG. 2, in the embodiments in which photo resist 26 is apositive tone photo resist, an over exposure is performed using a highlight energy. In accordance with embodiments of the present disclosure,when photo resist 26 is a positive tone photo resist, the light energyfor the exposure is adjusted and increased to a level that partialcross-links are generated in regions 26C, and regions 26A and 26B arenot cross-linked This is different from conventional photo lithographyprocesses, in which partial cross-links are not to be generated,regardless of whether a positive or a negative tone photo resist isused. Positive tone photo resists may have a characteristic that when itis over-exposed to a certain degree, partial cross-linking may occur insome over-exposed portions. As a result, the partial cross-linkedportions of the photo resist are not removed in the subsequentdevelopment of the photo resist. The remaining exposed portions of thepositive tone photo resist (which do not have partial cross-links) aredecomposed. Throughout the description, when a positive tone photoresist region is referred to as partially cross-linked, it means thatthe number of the cross-links generated therein is high enough to causethe respective photo resist region to remain after the subsequentdevelopment steps.

In some embodiments, the partial cross-linked photo resist 26 is apositive tone photo resist, and is not an ideal positive tone photoresist. The partial cross-linked positive tone photo resist 26 comprisesa polymer, and a weak cross-linking agent. When exposed to light, theunexposed portions of partial cross-linked positive tone photo resist 26behave like a general positive tone photo resist. The unexposed portionsof partial cross-linked positive tone photo resist 26 are notdecomposed, and will not be removed by the development step. The exposedportions of partial cross-linked positive tone photo resist 26 behavelike a general positive tone photo resist when exposed to light and theenergy of light is not high enough to generate cross-linking reactions.The exposed portions of partial cross-linked positive tone photo resist26 are decomposed, and will be removed by the development step. Whenexposed to light and the energy of light is high enough to generate thecross-linking reactions, the exposed portions of partial cross-linkedpositive tone photo resist 26 are slightly cross-linked, and will bepartially removed by the development step. The remaining thickness ofthe exposed portions of partial cross-linked positive tone photo resist26 is related to the completion level of the cross-linking reactions,which completion level of the cross-linking reactions is further relatedto, but not limited to, the energy of the exposed light, energydistribution of the exposed light, the reflection light from substrate,the refraction light and scattering light from neighborhoodenvironments, and the post-exposure bake temperature.

The needed light energy for exposing photo resist 26 depends on the typeand the composition of the photo resist. For example, an exemplarypartial cross-linked positive tone photo resist 26 is under a normalexposure (without generating partial cross-links) when exposed to thelight with an energy of about 10 milli-Joule/cm². When the energy of thelight is increased to about 30 milli-Joule/cm2, partial cross-linkingreactions occur in the exposed portions, and the remaining thickness ofthe exposed portions of partial cross-linked positive tone resist 26 mayonly be about 10% of the original thickness. When the energy of light isfurther increased to about 70 milli-Joule/cm², the remaining thicknessof the exposed portions of partial cross-linked positive tone resist 26may be increased to about 70% of the original thickness.

In alternative embodiments, the partial cross-linked photo resist 26 isa negative tone photo resist, and is not an ideal negative tone photoresist. The partial cross-linked negative tone photo resist 26 comprisesa polymer, and a weak cross-linking agent. When exposed to light, theunexposed portions of partial cross-linked negative tone photo resist 26behave like a general negative tone photo resist. The unexposed portionsof partial cross-linked negative tone photo resist 26 will be removed bythe development step. The exposed portions of partial cross-linkednegative tone photo resist 26 behave like a general negative tone photoresist when exposed to light and the energy of light is high enough togenerate and complete the cross-linking reactions. The exposed portionsof partial cross-linked negative tone photo resist 26 will not beremoved by the development step. When exposed to light and the energy oflight is not high enough, the exposed portions of partial cross-linkednegative tone photo resist 26 are slightly cross-linked, and will bepartially removed by the development step. The remaining thickness ofthe exposed portions of partial cross-linked negative tone photo resist26 is also related to the completion level of the cross-linkingreactions, which completion level is related to, but not limited to, theenergy of the exposed light, energy distribution of the exposed light,the reflection light from substrate, the refraction light and scatteringlight from neighborhood environments, and the post-exposure baketemperature.

In some exemplary embodiments, an exemplary partial cross-linkednegative tone photo resist 26 is under a normal exposure (with the allexposed portions are remaining after development) when exposed to thelight with energy of about 100 milli-Joule/cm². When the energy of lightis decreased to about 60 milli-Joule/cm², partial cross-linkingreactions occur in the exposed portions, and the remaining thickness ofthe exposed portions of partial cross-linked negative tone resist 26 maybe about 50% of the original thickness. When the energy of light isfurther decreased to about 20 milli-Joule/cm², the remaining thicknessof the exposed portions of partial cross-linked negative tone resist 26is decreased to about 10% of the original thickness.

The locations and the sizes in which the partial cross-links will begenerated are related to the topology of the photo resist, even if theyare exposed to the same light exposure energy. For example, as shown inFIG. 2, corner photo resist region 26C is a region that partialcross-links are likely to occur. Corner photo resist region 26C is atthe corner formed between top surface 20A of base structure 20 andsidewall 23 of features 22 and 24. Photo resist region 26A and 26B,which are over flat surfaces or surface portions of photo resist 26, areless likely to have partial cross-links generated, and are decomposeddue to the light exposure. Alternatively stated, the positive tone photoresist regions over flat surfaces need a relatively great amount oflight exposure (energy) before the partial cross-links can be generated,and the positive tone photo resist regions over high-topology surfacesneed a relatively small amount of light exposure (energy) before thepartial cross-links are generated. For example, assuming the lightenergy (measured as milli-Joules/cm², for example) used for exposingphoto resist 26 in a normal exposure step is En, then exposure energy2En is needed to cause partial cross-links to be generated in photoresist region 26C, while exposure energy 5En is needed to cause partialcross-links to be generated in photo resist regions 26A and/or 26B. Insome exemplary embodiments, a reference light energy is selected to bethe light energy used for exposing photo resist 36 (FIG. 5, when it is apositive tone photo resist). Accordingly, the energy for exposing photoresist 26 may be two times to five times the energy used for exposingphoto resist 36.

The needed light energy for exposing photo resist 26 depends on the typeand the composition of the photo resist. In some embodiments, the energyfor exposing positive tone photo resist 26 in a normal exposure (withoutgenerating partial cross-links) may be between about 10 milli-Joule/cm²and about 30 milli-Joule/cm². In accordance with some embodiments, toover expose positive tone photo resist 26 to cause partial cross-linksto be generated, the light energy may be between about 20milli-Joule/cm² and about 150 milli-Joule/cm², depending on the type ofphoto resist and the topology of the patterned surface.

In alternative embodiments in which photo resist 26 is a negative tonephoto resist, an under-exposure is performed using a low light energy.When the negative tone photo resist 26 is under exposed, partialcross-links may be generated in some portions of photo resist 26. Thepartially cross-linked portions are not removed in the subsequentdevelopment step. Other portions of photo resist 26 do not have enoughcross-links generated to cause it to be partially cross-linked, and canbe removed in subsequent steps. Similarly, the locations in which thepartial cross-links are generated are related to the topology of wafer100. For example, corner photo resist region 26C is more likely to havepartial cross-links generated than photo resist regions 26A and 26B. Inaccordance with some embodiments of the present disclosure, the lightenergy is adjusted and reduced to a level that partial cross-links aregenerated in regions 26C, and regions 26A and 26B are not cross-linked.

In some embodiments, the normal energy En for exposing a negative tonephoto resist 26 in a normal exposure (without generating partialcross-links) may be between about 100 milli-Joule/cm² and about 300milli-Joule/cm². It is realized, however, that the energy is alsorelated to the tool used. When the normal exposure occurs, the portionsof the negative tone photo resist exposed to light have polymer chainswith each other, and the portions of the negative tone photo resist notexposed to light do not have polymer chains cross-link with each other.In accordance with some embodiments, to under-expose negative tone photoresist 26 to cause partial cross-links, the exposure energy may be lessthan about 1 percent the normal energy En. In some embodiments in whichphoto resist 36 (FIG. 5) is a negative tone photo resist, a referenceenergy En is selected to be the energy used for exposing photo resist36. Accordingly, in some embodiments, the energy for exposing photoresist 26 may be less than about 1 percent the energy for exposing photoresist 36.

As a result of the partial cross-links, in the subsequent developingsteps, photo resist regions 26C remains, while the rest of photo resist26 are removed. The resulting structure is shown in FIG. 3A. Hence,photo resist region 26C fills the corner region, and form slope 32. Theformation of slope 32 reduces the abruptness in the transition betweentop surface 24A and top surface 20A.

The profile of the remaining photo resist region 26C is related to theenergy, For example, for a positive tone photo resist, if a certainexposure energy causes photo resist region 26C to have the profile inFIG. 3A, then increasing the exposure energy for a positive tone photoresist and reducing the exposure energy for a negative tone photo resistmay result in the profile shown in FIG. 3B, wherein FIG. 3B has morephoto resist left than in FIG. 3A. Conversely, reducing the exposureenergy for a positive tone photo resist and increasing the exposureenergy for a negative tone photo resist may result in the profile shownin FIG. 3C, wherein FIG. 3C has less photo resist left than in FIG. 3A.Accordingly, an optimum profile of photo resist region 26C (shown inFIG. 3A) may be achieved by experimenting different levels of lightenergy.

Furthermore, the generation of partial cross-links in photo resist 26may be achieved by adding cross-linking agent(s) into positive tonephoto resists or reducing the cross-linking agent(s) in negative tonephoto resists. In some exemplary embodiments, the exemplarycross-linking agent(s) include amino compounds such as melamine resins,urea resins, guanamine resins, glycoluril-formaldehyde resins,succinamide-formaldehyde resins, ethylene urea-formaldehyde resins, andcombinations thereof. In addition, by adding cross-linking agent(s) intopositive tone photo resists, the required minimum light energy for thedesirable over-exposure may be reduced, and by reducing cross-linkingagent(s) from negative tone photo resists, the maximum light energy forthe desirable under-exposure may be increased. Hence, the over exposureor under exposure is easier, and may be achieved using existing processtools.

Referring to FIG. 4, Bottom Anti-Reflective Coating (BARC) 34 and photoresist 36 are formed over the structure shown in FIG. 3A. BARC 34 isused to reduce the reflection from the underlying layers. In accordancewith some embodiments, BARC 34 comprises an organic material. Inalternative embodiments, BARC 34 comprises an inorganic material such assilicon oxynitride. Due to the formation of photo resist region 26C, thethickness uniformity of BARC 34 is improved compared to if photo resistregion 26C is not formed.

Photo resist 36 is then coated over BARC 34. In some embodiments, photoresist 36 is a positive tone photo resist. In alternative embodiments,photo resist 36 is a negative tone photo resist. Photo resist 36,regardless of being a positive tone photo resist or a negative tonephoto resist, may be selected from the same group of photo resists forforming photo resist 26. Furthermore, photo resist 36 may be the sameas, or different from, photo resist 26. Furthermore, photo resists 26and 36 may be both positive tone photo resists, both negative tone photoresists, or include on positive and one negative tone photo resist. Thethickness uniformity of photo resist 36 is also improved compared to ifphoto resist region 26C is not formed.

FIG. 5 illustrates the exposure of photo resist 36 using lithographymask 38, which comprises opaque portions 38A and transparent portions38B. In the illustrated embodiments, photo resist 36 is a positive tonephoto resist. The exposed portions 36A of photo resist 36, which arealigned to transparent mask portions 38B, are decomposed. In alternativeembodiments (not shown), photo resist 36 may be a negative tone photoresist, and lithography mask 38 may have a pattern opposite to what isshown in FIG. 5.

Next, as shown in FIG. 6, photo resist 36 is developed, and photo resistportions 36A (FIG. 5) are removed. The patterned photo resist 36 is thenused as an etching mask to etch the underlying hard mask 24 and targetfeature 22, as shown in FIG. 7. Patterned photo resist 36 may beconsumed, and hence are not shown in FIG. 7, although it may haveportions remaining after layers 22 and 24 are patterned. After thepatterning of target feature 22, photo resist 36, BARC 34, and hard mask24 are removed, and the resulting structure is shown in FIG. 8.

In the embodiments of the present disclosure, by forming an additionalphoto resist to smooth the topology of wafers, the BARC layer and thephoto resist for defining the patterns of the target layers haveimproved thickness uniformity. For example, as shown in FIG. 9, wafer100 may have a plurality of regions 40A, 40B, and 40C that comprisefeatures have different heights therein. Using the embodiments of thepresent disclosure, photo resist regions 26C are self-aligned to thegaps having different widths and heights, and the topology throughoutthe entire wafer 100 is smoothened. Since the pattering of theadditional photo resist does not require any patterned lithography mask,there is no overlay issue for aligning the lithography mask with theexisting patterns on the wafers.

In accordance with some embodiments, a method includes forming a firstphoto resist layer over a base structure and a target feature over thebase structure, performing an un-patterned exposure on the first photoresist layer, and developing the first photo resist layer. After thestep of developing, a corner portion of the first photo resist layerremains at a corner between a top surface of the base structure and anedge of the target feature. A second photo resist layer is formed overthe target feature, the base structure, and the corner portion of thefirst photo resist layer. The second photo resist layer is exposed usinga patterned lithography mask. The second photo resist layer is patternedto form a patterned photo resist.

In accordance with other embodiments, a method includes forming a firstphoto resist layer over a base structure and a target feature over thebase structure, and performing a first exposure on the first photoresist layer. A corner portion of the first photo resist layer ispartially cross-linked after the first exposure, which corner portion isat a corner between a top surface of the base structure and an edge ofthe target feature. The first photo resist layer is developed. After thestep of developing, the corner portion of the first photo resist layerremains, and the first photo resist layer is removed from the regionoverlapping the target feature. A second photo resist layer is formedover the corner portion of the first photo resist layer, the targetfeature, and the base structure. A second exposure is performed on thesecond photo resist layer using a patterned lithography mask. The secondphoto resist layer is developed to form a patterned photo resist.

In accordance with yet other embodiments, a method includes forming afirst positive tone photo resist layer over a base structure and atarget feature over the base structure, performing a first exposure onthe first positive tone photo resist layer, and developing the firstpositive tone photo resist layer. After the step of developing, a cornerportion of the first positive tone photo resist layer remains at acorner between a top surface of the base structure and an edge of thetarget feature. A second positive tone photo resist layer is formed overthe corner portion of the first positive tone photo resist layer, thetarget feature, and the base structure. A second exposure is exposed onthe second positive tone photo resist layer using a patternedlithography mask. The second positive tone photo resist layer isdeveloped to form a patterned photo resist. The target feature is etchedusing the patterned photo resist as an etching mask.

Although the embodiments and their advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the embodiments as defined by the appended claims. Moreover,the scope of the present application is not intended to be limited tothe particular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the disclosure.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps. In addition, each claim constitutes a separateembodiment, and the combination of various claims and embodiments arewithin the scope of the disclosure.

What is claimed is:
 1. A method comprising: forming a first photo resistlayer over a base structure and a target feature, with the targetfeature over and contacting a planar top surface of the base structure,wherein the first photo resist layer comprises a first portion over andcontacting the base structure, a second portion over and contacting thetarget feature, and a corner portion at a corner formed between theplanar top surface of the base structure and a sidewall of the targetfeature; performing an un-patterned exposure on the first photo resistlayer such that cross-linking is generated in the corner portion;developing the first photo resist layer, wherein after the step ofdeveloping, the first portion and the second portion of the first photoresist layer are removed and the corner portion remains; forming asecond photo resist layer over the target feature, the base structure,and the corner portion of the first photo resist layer; exposing thesecond photo resist layer using a patterned lithography mask; anddeveloping the second photo resist layer to form a patterned photoresist.
 2. The method of claim 1 further comprising: etching the targetfeature using the patterned photo resist as an etching mask; removingthe patterned photo resist; and removing the corner portion of the firstphoto resist layer.
 3. The method of claim 1, wherein the first photoresist layer comprises a positive tone photo resist.
 4. The method ofclaim 3, wherein the un-patterned exposure comprises an over exposure,and wherein partial cross-links are generated in the corner portion ofthe first photo resist layer.
 5. The method of claim 1, wherein thefirst photo resist layer comprises a negative tone photo resist.
 6. Themethod of claim 5, wherein the un-patterned exposure comprises an underexposure, and wherein partial cross-links are generated in the cornerportion of the first photo resist layer.
 7. The method of claim 1further comprising, after the step of developing the first photo resistlayer and before the step of forming the second photo resist layer,forming a Bottom Anti-Reflective Coating (BARC) over the target featureand the corner portion of the first photo resist layer.
 8. A methodcomprising: forming a target feature protruding above a base structure;forming a first photo resist layer over both the base structure and thetarget feature, wherein the first photo resist layer comprises: a cornerportion having a sidewall contacting a sidewall of the target feature,and a bottom surface contacting a top surface of the base structure; andadditional portions over and contacting the target feature and the basestructure; performing a first exposure on the first photo resist layer,wherein the corner portion of the first photo resist layer is partiallycross-linked after the first exposure; developing the first photo resistlayer, wherein after the step of developing, the corner portion of thefirst photo resist layer remains, and the additional portions of thefirst photo resist layer are removed; forming a second photo resistlayer over the corner portion of the first photo resist layer, thetarget feature, and the base structure; performing a second exposure onthe second photo resist layer using a patterned lithography mask; anddeveloping the second photo resist layer to form a patterned photoresist.
 9. The method of claim 8 further comprising: etching the targetfeature using the patterned photo resist as an etching mask; removingthe patterned photo resist; and removing the corner portion of the firstphoto resist layer.
 10. The method of claim 8, wherein the first photoresist layer comprises a positive tone photo resist, and wherein thefirst exposure comprises an over exposure.
 11. The method of claim 8,wherein the first photo resist layer comprises a negative tone photoresist, and wherein the first exposure comprises an under exposure. 12.The method of claim 8 further comprising, after the step of developingthe first photo resist layer and before the step of forming the secondphoto resist layer, forming a Bottom Anti-Reflective Coating (BARC) overthe corner portion of the first photo resist layer and the targetfeature.
 13. The method of claim 8, wherein in the first exposure, nolithography mask is used.
 14. A method comprising: forming a firstpositive tone photo resist layer over a base structure and a targetfeature, with the target feature protruding over the base structure,wherein the first positive tone photo resist layer comprises a firstportion over and contacting the base structure, a second portion overand contacting the target feature, and a corner portion at a cornerformed between a top surface of the base structure and an edge of thetarget feature; performing a first exposure on the first positive tonephoto resist layer, wherein the first exposure is an over exposure, andthe first exposure results in partial cross-linking to be generated inthe corner portion, and no cross-linking generated in additionalportions of the first positive tone photo resist that are directly overthe base structure and the target feature; developing the first positivetone photo resist layer, wherein after the step of developing, thecorner portion of the first positive tone photo resist layer remains,and the additional portions of the first positive tone photo resistlayer are removed; forming a second positive tone photo resist layerover the corner portion of the first positive tone photo resist layer,the target feature, and the base structure; performing a second exposureon the second positive tone photo resist layer using a patternedlithography mask; developing the second positive tone photo resist layerto form a patterned photo resist; and etching the target feature usingthe patterned photo resist as an etching mask.
 15. The method of claim14, wherein the first exposure is a non-patterned exposure without usingany patterned lithography mask.
 16. The method of claim 14, wherein thefirst exposure is performed using a first energy, and the secondexposure is performed using a second energy, and wherein the firstenergy is greater than about two times the second energy.
 17. The methodof claim 16, wherein the first energy is greater than about five timesthe second energy.
 18. The method of claim 14, wherein after the firstexposure, partial cross-links are generated in the corner portion of thefirst positive tone photo resist layer.
 19. The method of claim 14further comprising, after the step of developing the first positive tonephoto resist layer and before the step of forming the second positivetone photo resist layer, forming a Bottom Anti-Reflective Coating (BARC)over the corner portion of the first positive tone photo resist layerand the target feature.
 20. The method of claim 14 further comprising,after etching the target feature, removing the first and the secondpositive tone photo resist layers.